Cooling fans for heat-sensitive electrical equipment are frequently driven by single-phase induction motors having main and auxiliary windings with one or more capacitors connected in series with the auxiliary winding. Such motors, sometimes referred to as permanent split-capacitor motors because the capacitor is continuously in series with the auxiliary winding, are widely used in cooling fan applications due to their low cost of manufacture and starting ease. In many cases, it is necessary to provide an indication of cooling loss should the fan airflow become blocked by accumulation of dust or foreign objects. Mechanical airflow sensors such as vane switches are known, of course, but such switches require periodic calibration and are not particularly reliable. Accordingly, what is needed is more reliable and trouble-free apparatus for detecting airflow blockage.